Conventionally, as a material for the rotor to be used at high temperature portion of a steam turbine or gas turbine, 12 Cr ferritic steel having a low thermal expansion coefficient (e.g., C: 0.14%, Si: 0.05%, Mn: 0.50%, Ni: 0.6%, Cr: 10.3%, Mo: 1.5%, V: 0.17%, Nb: 0.06% and Fe: the balance) has been mainly used.
However, in recent years, in order to improve thermal efficiency, for example in a steam turbine, development has progressed so as to elevate the steam temperature to 650° C. or higher.
When the steam temperature is elevated to such high temperature, heat-resistant strength required for large-sized parts such as rotor also increases, so that conventional 12 Cr ferritic steel cannot be applied such requirement.
Thus, in view of material quality, materials having high heat-resistant strength at the higher temperature have been required.
For the material therefor, there has been known austenitic superalloys (e.g., A-286 (Cr: 15%, Ni: 26%, Mo: 1.25%, Ti: 2%, Al: 0.2%, C: 0.04%, B: 0.005%, V: 0.3%, Fe: the balance), Inconel 617 (Cr: 22%, Co: 12.5%, Mo: 9%, Al: 1%, C: 0.07%, Ni: the balance), Inconel 625 (Cr: 21.5%, Mo: 9%, Nb: 3.6%, Ti: 0.2%, Fe: 2.5%, C: 0.05%, Ni: the balance), or Inconel 706 (Cr: 16%, Ti: 1.75%, Al: 0.2%, Fe: 37.5%, C: 0.03%, Nb+Ta: 2.9%, Ni: the balance), which are excellent in corrosion resistance and oxidation resistance and have a excellent high temperature strength in comparison with 12 Cr ferritic steel.
However, they have an excellent high temperature strength but have a high thermal expansion coefficient, so that there is a problem that design flexibility is low.
All the parts constituting the steam turbine etc. are not necessarily exposed to 650° C. or higher and some parts are not required to have such high temperature strength, so that it is possible to use conventional 12 Cr ferritic steel for such parts.
In this case, it can be considered for the turbine structure to be assembled with 12 Cr ferritic steel and austenitic superalloys, but there is a possibility of inconvenience caused by a difference in thermal expansion.
In view of the application, austenitic superalloys with low thermal expansion coefficient have been disclosed in Patent Document 1.
Whereas a rotor of the steam turbine is extremely large and hence it is difficult to form whole structure with an austenitic superalloy. Therefore, a welded rotor in which plurality of rotor (disc) materials are produced and are subsequently integrated by welding is employed.
Consequently, materials for the combined welded rotor are required to have an excellent weldability.
In this regard, Patent Document 1 does not note any such weldability.
Moreover, although the above-mentioned individual rotor (disc) is smaller than a mono-block rotor, the welded rotor (disc) is also large and hence an excellent hot-workability is required for materials constituting the rotor (disc).
Patent Document 1: JP-A-9-157779